Today, LED backlighting is more and more widely adopted by monitors. For example, in LCD TV field, LED is tending to replace the traditional CCFL as the backlighting source. In backlighting applications, LEDs are arranged in multiple strings that are powered by a LED driver as shown in FIG. 1. In FIG. 1, the LED driver comprises: a voltage regulating circuit 101 configured to provide a power supply voltage Vout to the LED strings; and a current regulating circuit 102 configured to regulate each current flowing through an associated LED string of the plurality of LED strings. To provide a high enough voltage to the plurality of LED strings, the voltage regulating circuit 101 receives a plurality of feedback voltages FB1˜FBn of the LED strings, and selects a minimum feedback voltage to control the power supply voltage Vout.
In a LED system, when short/open fault happens in any one of the LED strings, a protection mechanism is needed to process the faulted LED string to ensure the proper function of the rest of the LED strings.
When an open fault happens, the conventional way is to pull the faulted LED string to a ground reference. In that case, the minimum feedback voltage would be near zero. After a control loop regulation, the power supply voltage Vout increases to high enough to trigger the OVP (over voltage protection). As a result, the voltage regulator stops working, and meanwhile the faulted LED string is removed from the control loop. In this way, the LED open fault protection is realized. When a short fault happens, the feedback voltage of the shorted LED string increases to trigger the short fault protection. However, the feedback voltages FB1˜FBn increase as the power supply voltage Vout increases when open fault happens too. The increase of the feedback voltage FB1˜FBn caused by the open fault may mis-trigger the short fault protection.
In some cases, when short to ground fault happens, i.e., a LED string is shorted to ground reference, the power supply voltage Vout increases too with the same theory analyzed above. In these cases, the short to ground fault may be misjudged as the open fault.